Frequency modulation system for receiving one of two overlapping waves



R. M. wlLMoTTE Sept. 2, 1952 FREQUENCYV MODULATION SYSTEM F'OR RECEIVING ONE O TWO OVERLAPPING WAVES Filed Jan. 24, 1950 2 SHEETS-SHEET l INVENTOR RAYMOND M. WIL MOTTE 7; ATTORNEY Sept. 2, 1952 2,609,494 FREQUENCY MonuLATIoN SYSTEM FOR RECEIVING oNE oF Two OVERLAPPING WAVES 2- SHEETS-SHEET 2 Filed Jan. 24, 1950 BY ATTORNEY Patented Sept. 2, 1 952 FREQUENCY MODULATION SYSTEM FOR RECEIVING ONE F TWO OVERLAPPING WAVES Y Raymond M. Wilmotte, Washington, D. C., as-

signor to Padevco, Inc., Washington, D. C., a. corporation of DelawareY Application January 24, 1950, Serial N o. Y140,243

7 Claims. 'l

This invention relates generally to apparatus and methods for the separation of signals overlapping in frequency, and more particularly to methods and apparatus for selectively separating two overlapping frequency modulated carriera v It is a broad object of the present invention to provide novel methods and apparatus for selectively separating two frequency modulated signals, which overlap in frequency, and which are of different amplitudes.

It is another broad object of the invention to provide a system for detecting modulation inherent on a first frequency modulated carrier in the presence of another weaker overlapping frequency modulated carrier, with substantially no interference from thelatter.

It is still another object of the present invention to provide a system for varying the phase of two overlapping frequency modulated carriers to such degree and in such sense as to eliminate phaseshifts of the sum of the carriers due to thel presence of the weaker of the frequency modulated carriers.

'The 'above andvstill further objects, features. 25

and advantages of the present invention will become apparent upon consideration of the following'description of a specific embodiment of the invention, especially when taken in conjunction with the accompanying drawings, wherein:

Figure 1 is a vector diagram of two overlapping frequency modulated carriers, of different amplitudes, which is utilized in explaining the theoretical basis of the present invention;

Figure 2 illustrates in schematic circuit diagram one mode of realization of the invention;

Figure 3 illustrates in functional `block diagram the essential elements of the invention, as illustrated in the schematic circuit diagram of Figure 2;

Briefly described, in the present invention, two

overlapping frequency modulated signals, one of' response to controlsignals, is capable'of shifting the phase of the superposed carriers. The out-'- put of the yphase shifter is then limited and detected in a frequency discriminator' in conventional fashion, andA if the phase shifting is properly carried out, the'output of the ultimate discri-minator detector will be the stronger of the' twoV signals, from which the weaker will be absent, or substantially absent, lor at least'verv materiallyreduced. j'v l y (Cl. Z50-20) is alsoderived, and the two signals are multiplied together, thus obtaining a resultant signal pro-v, poi-tional to Lthe product. It is further necessary I have foundlthat the phase shift, a, ofthe resultant vector R, relative to the vector of the strong signal, produced by the superposition of two overlapping frequency modulated carriers of ldR qdt

`where q is the beat frequency between the carriers, and the rate of change of phase is measured in radians pervsecond. This approximation is goodeven for values of a f En close to unity. Accordingly, if a voltage propor- 0 tional to v 1 i@ q dt is applied to the phase shifting network mentioned in the previous paragraph, to shift the I phase of a vector R in such sense as to reduce the shift in phase cau-sed by the weaker of the signals, the output of the phase shifting circuit containsl essentially only those phase and frestronger of the signals. A

In accordance with one embodiment of the lnvention, a signal proportional to is derived, a further vsignal proportional to dit di to derive al sensing voltage which determines 14,whether the frequency of the weaker signal w-l-q is above or below the -frequency of thequency variations due to the modulation of theirv stronger signal w, e., to determine whether lq is positive or negative -in respect to algebraic sign. The sensing voltage then establishes the V-sense in which the product function will be ap- 'plied to the electronic phase shifter, and therefore the algebraic sign of the phase shiftwhich is introduced by the phase shifter. i Thereby, the'l l product voltage having the proper magnitude Y "57,5Land sign, the necessaryvphase shiftsintheremay also-*becobtained by a process of division, i. e., by obtaining q and d@ di and carrying out the process Referring now more specifically to Figure 2 of the drawings, the reference numeral i denotes a source of two overlapping frequency modulated signals, E14-E2,v one of which, E1, may be assumed tobe stronger' than lthe other. The output of the source l may-be applied to a radio frequency amplifier and converter 2 in conventional fashion, for heterodyning of the resultant signal to an intermediate frequency suitable for amplication in intermediate frequency amplifier 3.

The output of the intermediate frequency amplier 3 is then a signal which may be represented by a vector R, equal at each instant to the sum of the vectors E1 and E2, representing the original two voverlapping frequency modulated signals, or.representing the beat envelope of the twozsignals, .as illustrated at The sum of the two vectors E1 and Ez may be represented completely by the expression R sin (wt--a) where R is the vector sum of E1 and E2, c the angle between R and E1, and w the frequency of the vector E1.

It is well known that if the resultant vector R beilimilted inaniplitude by means of a limiter, of the character conventionally utilized in frequenoy modulation receivers, the vector Ez may be substantially clipped from the vector R, leaving the vector E1, with distortion produced by the change Yin the value of a as the vector Ez moves relative to the vector E1. This distortion reaches a `rilasci-plu 111, Value when the vector Ez rotates .into phase'opposition with the vector Ei and passes through parallelism with the vector E1. At such times an extremely rapid change of phase angle of the vector R takes place, especially when the. ratio of amplitudes of E1 and Ez is close to unity. This rapid change of the angle is equivalent to a rapid change in frequency for ashort time interval. It will be clear then that the presence of the weaker signal causes periodic or recurrent short time frequency variations or frequency deviations, which constitute a distortion of the stronger signalE1,'by virtue of the presence of the superposed weaker signal E2. I have shown that the phase shift relative to the strong signal of the resultant vector R undergoes an angular rateofv change, given to a close approximation, by f 1` 1g A, c c E. df'

Ir avcltage proportional to q dt is ppliedto an electronic phase shifter, 5, after thebeat envelope 4 has been limited in amplitude in an amplitude limiter 6, the phase of the resultant voltage. R may be shifted in a direction and arnountvoppositefto that causing the disturbance, thereby eliminating the latter.

As a result the instantaneous frequency deviations previously mentioned do not occur, and the signal impressed on the limiter and discriminator 1, when detected therein, produces the desired modulation of thestronger signal free from interference dueto the undesired frequency deviations.

In order to accomplish the required phase shifting of rthe resultant vector R by means of the electronic phase shifter 5, it is essential, in accordance with the present invention, to develop a voltage proportional to irs q dt 1 This is accomplished. by developing a voltage proportlonalto il@ .dt and a further voltage proportional to l Q and multiplying the two voltages to produce a resultant voltage proportional to Lili g dt or by developing a voltage proportional tog-fand a` voltageproportional to 1 d@ Y dt anc` obtaining Ythe ratio,

To produce a voltagey proportional .to

e di

the. resultant voltage R is applied via, lead 8 to af detector `9 to obtain the envelope thereof, and,

thence to the controlelectrode ID of a pentode il,

which is anode loaded by means of .an inductance l2, The latter may be constructed to have a high inductance for the. frequency at vvhichRV varies in amplitude, and may be shunted with capacity 'i3 so as to carry nol appreciable R. F. voltage. Ac-

cordingly, there is developed across. inductance.

L a voltageproportonal to gli. dt

Thisfvcltage 1s applied via a D.c. bioekmgend sflgnaljcollplng condenser |74 `to a multiplier l5. The voltage R, i. e., the ,envelopel of superposed Waves E1 and Ea is further Suppliedto `a clipper l1 viara coupling condenser i8, so that at theinput ofi theA .clipper I1 is applied an alternating,l

voltage corresponding with the envelope of R.,

and having zero D.C,. components,A while atthe output of thefclipper .I'I' is generated' a, square Wave, or substantially a square wave,liavim-E: substantially no DC. component.. and lhaving positively going portions u. and negatively going portions -b,lwhi chs1ope relatively sharply; the total' square Wave being identifiable bythe referencev numeral I9'. The square wave l19is applied to,l

21 in series.

tive, and if conductive is arranged to have a very low internal resistance. Anode supply for the tetrode 2| is providedfrom a source 23 via a charging resistance 24, andthere is connected between the anode 25 of tetrode 2I and ground a charging condenser 28 and a by-pass condenser Accordingly, while the tetrode 2| is blocked the condenser 26 charges from the voltage source 23, the charging current flowing in the resistance 24 being substantially of saw tooth form. On the other hand, vwhen the tetrode 2| becomes conductive, due to its low internal resistance, the total voltage across the condenser 25 becomes substantially zero, since'l the condenser 26 is essentially short circuited, and the saw tooth voltage accordingly rapidly decays to substantially zero or ground value. Thefcondenser 26 remains discharged'while the wave form I9 is positive, i. e., from a to b, but upon reaching a point in the wave I9 such as is represented by b, a second charging cycle commences.

The voltage developed across the resistance 24 is transferred to a cathode follower 28, across the load circuit of which is then developed a replica of the saw tooth wave form corresponding with current flow in resistance 24. Since the wave form I9 consists of successive portions having positive and negative segments of unequal length or duration, the saw tooth Waves generated in the'load circuit of the cathode follower 28'- will similarly consist of saw tooth voltages of varying heights and spacings as indicated at 29. Itk

will, nevertheless, be true that each saw tooth Wave will have the same slope as any other saw tooth wave, since in each case the time constant of the charging circuit comprising resistance 24 and condenser 2,6 remains unchanged. It will further be true that each saw tooth wave will commence to build up at times b of the wave form I9 and will decay to zero at times a of the wave form I9. Accordingly, the maximum height which is reached by any of the saw tooth wave forms comprising the wave 29 is proportional to l1 since the higher is q, the shorter is the time interval between points a and b of wave form I9, or between points b and a. of wave form I9. It then becomes necessary merely to obtain a measure of the peak values of the saw teeth of the wave 29 in order to obtain a measure of I have accordingly provided circuits for sampling the wave 29 at the peak of each saw tooth thereof. To this end I apply the wave form I9 at the output of the clipper I'I via a coupling condenser 30 and a phase reversing tube 3I to a shock excited pulser 32. The latter comprises a triode 33 having a grounded cathode 34, a control electrode 35 and an anode 36. The anode 36 is connected to a source of B+ voltage'3'I via' a choke 38, which is by-passed by a damping resistance 39, and which includes some inherent capacity 40.

The electrical constants of the inductance 38 and its associated damping resistance 39 and capacity 40 are such that the circuit is critically damped, thus to produce essentially one-half cycle upon sudden discharge of the energy stored in inductance 38. The operating frequency of before.

the circuit being approximatelyzl ,megacyclarthe pulse produced bytheidischargeof the inductance 38 lasts for a timeinterval of the order of 1 microsecond,` taking the form illustrated at P,

and the positive peaks occurringk at points a.

The control electrode 35 is biased by applying which may be assumed, nevertheless, to have inherent internal resistance between the cathode 34 and the control electrode 35. The internal resistance, when considered in series with. resistance 4I, constitutes a voltage divider circuit, so .thata .denite proportion ofthe B+ voltage is impressed between control electrode 35 and cathode.

34, and so that the latter is normally positive.

Due to the presence of the phase inverter 3Ithe' Wave form I9 as applied to the control electrode 35 consists of negatively poled portions subsisting from point a to point b, and positively poled portions from point b to point a. Duringvthe positively poled portions the triode 33 is conductive and current flows in the inductance 38. Upon effecting a transition from positive to negative, i. e., at the points fa of the waveform I9, current flow in the inductance 38 is suddenly terminated. since the triode 33 becomes non-conductive. The energy storedA in the inductance 38 then discharges, as vhas been explained hereinance for transferring the voltage generated bydischarge of the inductance 38 to the control,

electrode 45 of a double triode 46, connected with. the separate triode sections in back-to-backrelation. Additionally, the control electrode .45,is connected Yto a source of bias Voltage,fi7, whichA serves normally to cut olf the triode sections. The voltage impressed across the coupling resistance 44 by discharge of the inductanceV SIS-isv of sufficient magnitude to render the triode ,sec-

tions conductive. One cathode 50 and aycorrespondinganode 5I of the double triode 45 are tied together and connected to the positive end.-

of the load resistance of the cathode follower 28, and accordingly have impressed thereon the Wave 29. Nevertheless, since the double triode 46 is normally non-conductive the Voltage represented by the Wave form29 causes no current tov iiow through the double triode. However, kwhen the gating voltage developed across resistancefM'. is impressed on the control electrode' 45 ofthe double triode 46, current flows therethrough in responseto the then voltage of the wave form 29. The peaks of the wave form 29occur at times a of the waveI9, and it is lprecisely at these points a that the inductance 38 discharges and applies.

a positive gating pulse vto the doubley triode 46, causing the latter to` become conductive. A

pulse of current then flows through the double triode V45, charging the condenser 52 with its ungrounded end positive, and the total charge of the condenserV 52 is such that the voltageacross the condenser 52 is substantially the voltage of the wave form 29 at the peak of a saw-tooth. The double triode 45 cutsoif immediately; the gatingpulseceasea .andthecharge '1m-r The discharge requires anr extremelyl pressed on the condenser .52 remains trapped; until a. succeeding: .gating pulseoccurs When. this. occurs. the. then. existing saw-tooth of wave. 29i may be larger or smaller than, the preceding saw-tooth. Ii larger, additional voltage: charge is,`- added to the condenser 5.2, hoesting its. voltage; accordingly. If smaller,y the .condenser 52 discharges. through the double. triode 4o via .the anode; 53V and cathode 5H.. and the.- total voltage across. the condenser 52 Vaccord-- mgl-y decreases. It will then, oe seen that the; condenser 52. maintains continuously a. voltage mbstantiallyv equal. to the. maximum voltage ci the vlast saw-tooth of the wave 29- The. voltage available vacroSs the condenser 52` is applied to an input circuit of the. voltage. multiplier l5, and. since the peaks of the saw-teeth. of the. Ywaive 29v reach. values proportional to ,il so the voltage applied to the voltage multipler i5 and derivi-ngy across the condenser 52 similarly is at all times proportional to The. voltage multiplier I5, multiplies. the. two voltages applied thereto, one of which is. proportional to @i di.

and the; other toA and Vin its output circuit provides a product voltage. This product voltage is impressed on a phase reversing circuit 54 which is controlled inv turny by a sensing circuit 55. The latter isv controlled to provide a sensing voltage in response to the wave form R, which is applied thereto via lead 56 from I. F; amplifier 3. The circuits 54` and 55 correspond to those disclosed in-my application for U. S. patent Serial #133,871, entitled Fi M. System I, led December 19, i949. Reference is made particularly to Figure- 4 of that application, wherein is shown a gating wave generator control circuit 8 and a gating wave generator 24, which constitute the sensing circuit 55 of the present application, and whichl generate a gating wave which appears on the leadL 51 in accordancewith whether the frequency of theweaker signal VEsci the two signals E1 and E2 is higher or lower in frequency than is the stronger signal E1. The reversing circuit 5ftA of the present application corresponds to the phase inverterl 3l and the polarity correction gating circuit 3Uv of Figure 4 of my application, Serial #133;8'71, led December 19, 1949, and serves accordingly to determine the phase of` the output of the voltage multiplier i5 as itr appears atV the output of the polaritycorrection circuit 54 in accordance with the sensing voltage provided by the sensing circuit 55'. It will` hev recalled that in the above-identifiedprior` application it was the phase of a `beat frequency1 as provided by beat frequency detector and discriminator 4 whichr was tor be corrected. In the present application, however, it is Vthe output of a voltageI multiplier I5 which is to be corrected. In either event.. the circuits operate identically, and detailed circuits corresponding with` the. block d-V all).I

mediate frequency amplifier 3.

agrexn. of Figure 4,. as well. as a full exposition, ofthe principles which. those' circuits-opere. ate., is provided in. the aforementionedapplica--v tion, and its provisionA herein would therefore be repetitious. l

The output of the polarity correction. circuit 5.4 is applied viaa lead 58 to phase shifter 5y and servesl to shift the phase, of the signal at, the. output thereof with respectv to the phase of. the. signal4 at; the. input thereof. The output of the.- voltage. multiplier l5v is of. proper magnitude to reduce the phase. between the resultant. vector R1 and the vector representa-tiveI of the stronger sig;- nal; E1 to zero, provided the voltage is. of proper: polarity to eiect a phase. shift in the, proper direction. The polarity of` the; voltage. applied over; the lead 5.8 is controlledin accordancewth the directior-x` of rotation of the vector E2 by the polarity circuit 54. and the sensi-ug: circuit 55.. Accordingly, the output ci' the. limiter` and dis-- crimi-nator 'I will consist of the stronger signal E1 Without any distortion due to phase shifts.l caused by the presence of the weaker signal, and accordingly without; distortion caused by' sudden. frequency shifts of the resultant, of the two over. lapping signals., v

llssentially,l in accordance. withtheV present. invention, the resultant vector R. remains-al-Y Ways; in phase with; the strongery signal- Ei at the output of thephase. shifter 5, and theipresence, of the weaker signal; does not, distort the stronger signal respect to frequency variations thereof.

Reference now made to Figure 3 of: the; accompany-ing, drawings wherein is illustrated; iniunctional block diagram a system correspond ing withA the detailed circuit diagram oi-Figure 2 of the accompanying drawings., It. jwill' be, perceived that the two overlapping frequencyl modulated waves En, E2 proceed from a source of such. waves., such as. an antenna or transmis.-` sion line, to a radio frequency' amplifier and converter 2, which` provides at its output anA inf-- termediate frequency signal` corresponding with superposed or overlapping signals EF1-and. E2. these latter then being amplified in anA inter The output of the intermediate frequency amplier is applied to a limiter 6, which clips the wave E2, assumed to be the weaker of the two signals,A trornthe wave E1, leaving the wave. Er, with,` however.,l phase shiftsy introduced by the presence of the. Wave Ez. Otherwise stated, the; output of the limiter 6, corresponds with the;- resu-ltant vector- R, after thee latter has been clipped to removeamplitude variations thereof due to the superposition of the vectors Er-and E2. It will be noted that a phase angle exists between the vector R and the vector E1 which is continually varying hothu in` algebraic sign andin magnitude, since. it is. caused by the original superposition of the; two. wavesV E1 and E2,A which. may be: assumed in. general to be unrelated.v

In accordance; with the present invention the, output of the. I. E. ampliiier-3 is applied tot a. device. |108; whichA generates a voltage propor-v tional tof where q is the frequency diiierence between the two voltages E1 and Ez. The polarity of the; output of the4 device lili is always positive; as will' be evident from a discussion of the system or Figure 2', regardless of' the algebraic sign or require.

2, wherein the voltage q. There is further developed in a generator Inl avoltage proportional to dt .Y

which may be derived by a simple differentiation process from the wave R. The output of the generator IUI is of random polarity. The output of the generator I l is applied to a multin plier I which provides at its output a voltage proportional in amplitude to 1 di y' It will readily be realized that it is essential to introduce, in order t0 compensate for phase shifts for the vector R which are due to the presence ofA the signal E2, compensating phase shifts of ,either polarity as the instantaneous situation may From this fact alone it will be realized that the output of the multiplier l5 is not suitable as it stands, for performing the function required `of it. The output of the multiplier E5 is of proper amplitude at each instant to remove the phase deviations of the Vector AR due rto the presence of the vector E2 when applied to the electronic phase shifter 5, but is not necessarily of proper polarity. The polarity of the voltage which is required to perform the phase shifting function is, however, determined. by the algebraic sign of q alone. This is true since the input to the voltage multiplier I5 as it is derived from the generator lill represents d R dt in both magnitude and algebraic sign. That this should be so is evident upon slight consideration of the detailed circuit diagram of Figure is derived by diiferentiating the voltage R, and passing the resultant to the voltage multiplier via a condenser I4, which removes the D.C. component of the output of the differentiating pentode and accordingly passes to the voltage multiplier I5 an alternating Voltage. On the other hand, the voltage applied to the voltage multiplier l5 from the condenser 52, and which represents isalways positive. Accordingly, any error in algebraic sign of lead |05 may then be applied to the electronic phase shifter 5 to produce the necessary phase different rates. y

While I have described one specific, embodiment of the invention it will be clear that .varia- -tionsthereon in both general arrangementand specific detail, vmay be resorted to without d ee parting from the true scope and spirit off the invention as dened in the appended claims.,

What I claim and desire to secure by Letters Patent of the United States is:

1. In a frequency modulation receiving system, for receiving the stronger of two overlapping frequency modulated waves` and for reducing in,-

terference dueA tothe weaker of said signals, where R represents the instantaneous amplitude of the. envelope of the overlappingwaves', and where q represents the frequency of said envelope and ,the` difference in frequency of said waves, means for generating a .voltage proportionalto dR` f Y 7d?" means for generating a further voltage proper tionalto 4 `q Y. means for multiplying tha-amplitudes,offfsaid voltages to derive a-*product voltage, andmeans for shifting the phase of said overlappingwaves in proportion to said product voltagegandina sense determined by the algebraic sign of q. 2. In a frequency modulation receiving system for demodulation of vone of two superposed waves E1=sin ',wt. and E2==a Y sin :iw-l-q) t; vwith substantial reduction ofr interference duetoezthe other, where w is ra varying frequency andiw-l-q is a second overlapping varying frequency, Where a is a fraction less than unity Iand equal to and where E1+E2=R sin (wH-a), by reduction of a, means for developing a voltage proportional to a, and means for varying the phase of superposed waves E14-E2 in response to said voltage in such sense and in such degree as to reduce a substantially to raero.

3. In a frequency modulation receiving system for demodulation of one of two superposed waves E1=sin wt and E2=a sin (w-l-q) t, where w and w-l-q are frequency modulated waves, where a is a fraction less than 1 and equal to f rotations at 'acuda-oa 11 fand where the .superposed Awaves :may be represented as E1+E2;R.sin(wt+a9 the `clomblnatioirl comprising, `a v'source c'fsupecposed waves E14-Ez, .an amplitude limiter for said superposed' waves, fan electronic :phase shifter, and -a discriminator, all cascade, and means for continuouslyv controlling `said electronic phase shifter to `reduce angle -a substantially to zero.

v5. In -a system Vfor demodula'ting two frequency modulated `waves Ei=sin wt and E2=a sin (1o-Pq) t, where w is `the frequency of one of said waves, -q the beat frequency between said waves. atheratioof El' Awhere lthe super-posed waves may be repreis fa function of q, means for multiplying togethersaid ,rst and said further voltage waves to obtain a product function voltage wave propcrtionalto nl@ qdt at every instant. of time, means for selectively :reversing'thepolarlty of said product yfunction voltage wave inresponse `,to `said means ior sens- 12 ing, means .for `applying saidvr product tfnnction voltage, after selective polartyireversal by said means for selectively reversing, to control said electronic phase shifter.

6. In a frequency modulation receiving system `for one of two superposed waves E1=sin wt land Ez=a sin ('w-l-cpt, where w and wl-qare frequencies of said frequency modulated waves, where a is a fraction less than 1 and equal to and where E1 and Ez=R sin (wt-l-e), a being the angle between E1 and E2, means for developing a voltage proportional `to .g dt and means vresponsive to said voltage for 'modi'- lying the angle yai by shifting the phase `of said superposed waves.

'1. 'In combination, means for receiving two overlapping frequency modulated waves, means for developing control signals in response to said `two overlapping frequency modulated waves, and

means responsive to said control signals for changing the phase of the sum of said two Yover-- lapping frequency modulated waves continuously to the phase of the stronger one of said two overlapping'frequency'modulated Waves. y

RAYMOND M. WILMO'TTE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED kSTllJI'ES PATENTS Number Name Date 2,259,000 Nyquist Oct. 1.4, 1941 .40 2,371,416 Tuniek Mar. 13, `1945 2,480,820 Hollingsworth Aug. 30, 1945 

